The simple answer to this is that that flail is accidentally engine-flagged as being a "plot item".
It actually
isn't, of course - so either it was a part of some miniquest that was scrapped from the game before it went public (unlikely) or it is simply due to a small developer mistake (very likely, especially since the toolkit lists it as a part of the standard item list and not as a part of the quest-item one).
-Zel